A police helicopter was hovering above the scene while uniformed officers patrolled the area.

Scotland Yard had earlier moved to avoid travel chaos in London by imposing conditions on the demonstration, including limiting it to an hour.

Organisations including the Rail and Maritime Transport union (RMT), London Cab Drivers Club (LCDC) and Licensed Taxi Drivers Association (LTDA) are all represented at the protest in Trafalgar Square.

Derrick Hoare, of the LTDA, said they wanted to highlight the length of training -, between four and seven years - taxi drivers undergo before being licensed.

The RMT's Ian Beetlestone said he hoped the demonstration would send a message that Uber is "operating in a grey area".

He said: "We are not objecting to competition. We have had competition for years from minicabs but we haven't caused gridlock over it.

"We have to jump through hoops to be regulated and we don't feel people involved in these new apps are being subjected to the same regulations."

Tourists have been caught up in the protest. Eric Radcliffe, a retired nurse visiting the capital from Manchester said he agreed entirely with the demonstration.

"I wouldn't let my daughter take a taxi booked through an app," the 62 year-old said.

Rejecting the claim from both Uber and Transport for London (TfL) that all drivers are regulated he said: "I don't believe it. This protest here today is what democracy is all about."

But Uber's general manager Jo Bertram claimed that "London wants Uber in a big way".

She said: "Unsurprisingly, the LTDA, which is stuck in the dark ages, is intent on holding London to ransom and causing significant economic impact to Londoners today, estimated to be £125 million.

"We join Transport for London and the Metropolitan Police in calling on the London Taxi Drivers Association (LTDA) and others to call off this unnecessary and irresponsible strike.

"We passed TfL's most stringent and comprehensive audit of a Private Hire Vehicle operator to date, passing with flying colours."

Uber is one of a number of apps people can use to book and pay for taxi journeys.

Launched in 2009 and currently in operation in more than 70 cities across 37 countries, the app makes cities more accessible for people by "seamlessly connecting riders to drivers", according to the company website.

The app detects the GPS on a mobile device and finds the nearest available driver. Users can track the vehicle, pay with their phone, and even split the fare with friends using a credit card system.

Legal action is being taken against Transport for London (TfL), but drivers are taking direct action, starting with today's protest. Motorists have been advised to avoid central London during the protest.

The Metropolitian Police inposed restrictions on the number of people who could take part in the demonstration and limited it to an hour after saying "repeated attempts" to contact organisers for constructive talks had failed.

Organisers deny being unavailable to talk.

London mayor Boris Johnson said he understood the concerns of taxi drivers but added that the issue should be resolved in the courts.

He said London's black cab trade was crucial to the fabric of the city, adding: "The hackney carriage is iconic, and it is absolutely right that our cabbies are properly protected.

"Indeed it is enshrined in law that only black cab drivers have the right to be hailed from the street, and that principle, the bedrock on which our black cabs operate, will never change.

"Black cab drivers are the face of London not just for Londoners but for millions of visitors to our city. Their licence is dependent on passing 'the knowledge' - years of hard graft go into learning every street name, and memorising every conceivable route across that city. That knowledge is why people choose to use black cabs.

"There must, however, be a place for new technology to work in harmony with the black cab, and we shouldn't unnecessarily restrict new ideas that are of genuine benefit to Londoners.

"Even so, the recent emergence of a range of new apps has raised some important questions in relation the operation of the private hire and taxi trades."

London student Salma Kahn, whose elder brother runs a London cab firm, said the introduction of apps like Uber is a major concern to small taxi businesses.

The 30-year-old, who came across the protest in central London during a day trip with her mother, said he is "very worried" by it.

She said: "My brother runs a taxi firm in Kensington and he has mentioned his concerns over these apps.

"He said the worry is for the local workers. If an American company comes to take over where will those people go?"

An RMT representative in Trafalgar Square used a megaphone to call for the protection of jobs.

He told those gathered: "Do not deregulate. This is about our future, our families' future."

Roads around Trafalgar Square were closed to non-taxis and t raffic lined nearby streets around the area during the hour-long demonstration.

Many motorists chose to sit on the pavements as they waited for the protest's end.

A London minister said his wife had to leave work early to pick their four-year-old son up from nursery after he got stuck in a jam for almost two hours.

Twenty-eight-year-old David Batista said he was frustrated at the delay, although he added that he received an email alert from TfL about the disruption but forgot about it.

"I didn't expect this," he said. "I would've taken a different route, had I known. It's caused me quite a lot of disruption. I've had to cancel a meeting this evening that I won't make now."

Delivery driver Sebastiano Conte said the protest delayed him getting home but added that he sympathised with the taxi drivers.

"It's a legitimate protest," said the 47-year-old from Watford. "Yes, it's caused me hassle but I understand where there're coming from."

Almost 30 minutes after the protest was due to end taxis still lined the roads around Charing Cross, beeping their horns continuously.

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